Not Applicable.
1. Field of the Invention
The present invention includes bainitic steel doctor blades, bainitic steel coating blades, bainitic steel creping blades and bainitic steel rule die knives ed in gravure printing, flexographic printing, paper making, die cutting of materials, such as, paper, plastic, foam, leather, etc. Other uses include printing processes such as pad printing and electrostatic printing, glue application arid other uses which will be apparent to those skilled in the art. This invention also relates to the process for producing bainite strip steel.
2. Discussion of Related Art
Various commercial industrial processes require metallic components that have extremely high straightness characteristics, high wear resistance and, in some cases, are also capable of being bent around small radii of bending. These components include doctor blades, used in such processes as flexographic and photogravure or gravure printing. Flexographic printing, formerly called analine printing, comprises a method of rotary printing utilizing flexible rubber plates and rapid drying fluid inks. Gravure printing is a printing technique wherein intaglio engravings of an image which are to be printed on a substrate, such as paper, are formed by known techniques on the surface of a gravure cylinder. Intaglio engravings are those where the elements to be printed are below the surface of the gravure cylinder, having been cut or etched into the metallic cylinder to form ink retaining cells. During printing, the gravure cylinder is immersed in fluid ink. As the cylinder rotates, ink fills tiny cells and covers the surface of the cylinder. The surface of the cylinder is wiped with a doctor blade, leaving the non-imaging area clean while the ink remains in the recessed cells in the cylinder. A substrate, such as paper stock, is brought into contact with the image carrier with the help of an impression roll. At the point of contact, ink is drawn out of the cells onto the substrate by capillary action.
Rule die knives are used in the cutting, creasing and perforating of various substrates such as, paper, cardboard, plastic, leather and foam.
Coating and creping blades are used in the manufacture of paper of various types wherein the blades are used to separate paper webs from calendar surfaces and used to apply coatings to the paper stock. Coating blades are also used to apply coatings, glue and protective films to a variety of substrates used in many different industrial processes.
While commercial tolerances of strip steel may generally have a straightness, referred to as camber, of about 0.375 inch per four feet, doctor blades and rule die knives used in the flexographic and gravure printing processes require a camber of a maximum of about 0.040 inch per ten feet and preferably about 0.024 inch per ten feet. This requirement is nearly one-hundred times more stringent than the tolerances in commercially supplied strip steel. Presently, there are very few manufacturers, none of which manufacture in the United States, that produce strip steel for the manufacture of these products. As a result of the limited suppliers and their foreign residences, these components are not only expensive, but are also susceptible to periods of unavailability.
In addition to low tolerances for straightness, it is desirable that doctor blades and rule die knives have relatively long useful service lives. Gravure and flexographic printing equipment are universally recognized to be expensive, and the labor costs associated with running these printing operations are significant. Printing pressmen are highly skilled and command high labor costs. It should readily be appreciated that anytime a gravure press or flexographic press is not operating during periods when it is supposed to be producing a printed substrate (downtime), significant costs are expended. Such costs are not likely to be recouped. Downtime may also result in the failure to meet printing deadlines. Thus, it is highly desirable to use doctor blades and rule die knives that require as few replacements as possible because such components can only be replaced during downtime.
These components are presently made of high carbon steel containing about 0.80% to 1.25% carbon by weight that is hardened and tempered to a martensitic structure. Martensite, a very hard and brittle microstructure in steel, has a fine, needlelike appearance under a microscope. While there is some correlation between higher hardness of this type of steel and better wear resistance, there is a limit to increases in hardness of martensitic steels to improve wear resistance due to the added brittleness that accompanies higher hardness. A practical limit of 54 Rockwell C is generally acknowledged, above which the parts become too brittle for use in printing press applications. A hardness of Rockwell of 48-52 Rockwell C is preferable.
Factors that contribute to the wear of doctor blades include a combination of abrasive wear, adhesive wear and wet impingement wear. Depending on the specific application any one or more than one of these types of wear may significantly contribute to reducing the wear life of doctor blades.
Attempts to improve wear properties of these components have included coating the wear surface with metallic materials such as chromium and non-metallic materials such as TiN, diamond, nitrides, SiO2 and sprayed ceramic. There also has been some use of edge hardening on alloy steels. While these methods improve wear resistance, they are expensive to apply and do little or nothing to change the camber. In certain instances, these processes actually can be deleterious to camber due to the high temperatures encountered in the particular process causing stress relief or thermal distortion,
In attempts to solve some of the technical problems associated with martensitic steel, the a use of cold rolled eutectic carbon steels with tensile strengths in excess of 300,000 psi has met with some success in gravure printing with water based inks. Cold rolled austenitic stainless steels were used for some time, but have been replaced by martensitic stainless steels.
Some have offered alloy steels and special high carbon steels such as SAE 52100, but these alternatives still contain martensitic structures. These special high carbon steel components therefore have the drawbacks of being expensive and/or show little improvement in useful wear life. Notably, none of these martensitic steels have answered the problem of long-term camber being greater than desired.
Coating and creping blades used in paper manufacturing have similar requirements to those of doctor blades. Because these blades are usually made of thicker material in the range of 0.024-0.060 inch there seems to be less problem with camber, but wear problems persist.
Rule die knives have requirements similar to those of doctor blades in that they must be very straight and durable. They must be sufficiently hard to permit edge sharpening, and they also must exhibit good sharpness retention when used to cut abrasive materials including kraft paper, coated stock and abrasive plastics. In addition, however, rule die knives also must be capable of being bent with small radii of bending.
In the past, this requirement has been met by various means including employing a softer metal, hardening the cutting edge, decarburizing the outer surfaces of the blade to depths of 0.003-0.006 inches, laser hardening of the cutting surface only and induction hardening after bending. All of these means are expensive.
It is believed that martensitic steel has not been successful with respect to camber requirements of doctor blades and die knife blades because of distortions that occur as a result of the austenitizing, quenching and tempering operations used in manufacturing the martensite. Quenching is the rapid cooling process in which the heated steel is plunged into a liquid or other medium to harden the metal. The heated steel, which has a temperature in excess of 1400xc2x0 F. and is in austenite form, is rapidly cooled to room temperature, changing from austenite to martensite.
Because martensite is very strong and hard, yet very brittle, it is generally tempered. Tempering involves reheating the quenched steel to a temperature that is below the steel""s lower transformation temperature to increase ductility and relieve stress. The lower transformation temperature is the temperature at which the formation of austenite begins. Relief of rolling stresses in the metal, thermal distortion during heat up, metallurgical structural changes with resulting changes in dimensions together with quench distortion all contribute to the camber problem.
Martempering and austempering have been used to address some of the distortion and dimension problems. These two alternatives involve heat treatments interrupted by cooling operations rather than quenching to room temperature.
Martempering is a process where steel, heated to the austenitizing temperature, is quenched to an intermediate temperature above the martensite start temperature, Ms, and held at that temperature for such duration that the temperature of the entire material is equalized. When temperature equilibrium is established, the steel is then slowly reduced in temperature, to room temperature. During this period, there is a generally uniform transformation from austenite to martensite throughout the cross section of the steel. This process produces steel with a microstructure of untempered martensite. It is very brittle and highly stressed. To regain toughness and ductility so that this steel can be used in mechanical operations, it must be tempered back resulting in some reduction of hardness and ultimate strength.
Commercial heat treating lines of the type used to manufacture steel for doctor blades use a form of martempering wherein the temperature is first reduced from austenitizing temperature by a rapid quench into either molten lead or molten salt at a temperature above the Ms. The steel is then removed from the quench medium and air cooled to room temperature before it is heated again to perform a tempering operation on the untempered martensitic steel. The temperature of the quench media is not critical so long as it is above Ms and well below the knee of the Time-Temperature-Transformation (TTT) curve, thus preventing the formation of pearlite which contains a softer microstructure than does martensite.
Austempering is a process that involves heating the steel to austenitizing temperature, then quenching it in lead or salt to a temperature above Ms and then holding it for about twenty minutes to two hours at a specific temperature selected for the steel composition and desired hardness. During this holding time, the steel structure changes from austenite to bainite, a specific microstructure different from martensite. The bainitic microstructure consists of ferrite crystals and dispersed carbides formed from the austenite produced by the high temperature austenitizing. The isothermal hold time permits the carbon atoms to diffuse to form carbide crystals, leaving the surrounding ferrite low in carbon content. In contrast, when austenite transforms to martensite, there is insufficient time for carbon atom diffusion and consequently martensite is supersaturated with carbon atoms trapped between the iron atoms. This creates high stress, distortion, and an increased tendency to brittle fracture. Also contributing to the latter characteristics of martensite, is a high density of crystal imperfections within the martensite caused by the quenching and diffusionless transformation process.
The ferritic matrix of bainite produced by isothermal transformation, in addition to the absence of carbon atom supersaturation, has a much lower density of imperfections and therefore reduced internal stresses and reduced sensitivity to brittle fracture as compared to martensitic steel. Thus, tempering is not required for bainitic microstructures, especially in high-carbon steels in which high hardness and wear resistance are required. Following the bainitic conversion, the material is cooled to room temperature. No further operations, such as tempering, are required. There is generally less distortion of material, i.e. less dimensional change in the microstructure size and density as compared to the conversion to martensite due to the more gentle conversion to bainite.
The austempering process provides less distortion, i.e. less dimensional change in size or density of material as compared to the conversion to martensite due to the more gentle conversion to bainite in the bainite process, and the elimination of the tempering operation which is to some extent a stress relieving operation. The obvious disadvantage to the austempering process is the long holding times at a precise temperature. For heat-treating individual parts, this limitation is not too severe. For continuous strip production, however, the cost of the large holding time and area, as well as the low production rates make the process commercially uneconomical.
It should be noted that many of these processes use anti-friction bearings to move or turn the steel during manufacture. Anti-friction bearings are defined herein to be bearings that replace sliding friction with rolling friction and include ball, needle, roller and tapered roller bearings Conventional anti-friction bearings are subject to very short useful lives because of environmental conditions. The combination of oxidation of lubricants, tempering, abrasive oxides from the strip, dimensional changes during heating and cooling and seal failure contribute to very rapid destruction of these bearings. Commonly available bearings typically do not last more than a few hours at temperatures that may be as high as 650xc2x0 F. In some cases, failure occurred in a single run causing bearings to seize and damage to the strip being processed.
What is needed is an improved doctor blade for use in printing operations.
What is further needed is a doctor blade that exhibits high straightness and low wear.
What is further needed is an improved doctor blade that has a good working life.
What is further needed is a doctor blade that minimizes press downtime,
What is further needed is a doctor blade that is economical in cost.
What is further needed is a doctor blade comprised of steel wherein the steel microstructure is substantially all in bainitic form.
What is further needed is a doctor blade comprising a carbon steel and at least one alloying element selected from chromium, vanadium, manganese, tungsten and niobium wherein the microstructure is substantially bainitic.
What is further needed is a doctor blade comprised of high carbon steel having a bainitic microstructure wherein the carbon content is generally within the range of 0.70% to 1.25% by weight.
What is further needed is an improved rule die knife for use in cutting operations.
What is further needed is a rule die knife that exhibits high straightness, low wear and is capable of being bent around a small radius.
What is further needed is an improved rule die knife that has a good working life,
What is further needed is a rule die knife that minimizes machine downtime.
What is further needed is a rule die knife that is economical in cost,
What is further needed is a rule die knife comprised of steel wherein the steel microstructure is substantially all in bainitic form,
What is further needed is a rule die knife comprised of high carbon steel having a bainitic microstructure wherein the carbon content is generally within the range of 0.70% to 1.25% carbon by weight,
What is further needed is a rule die knife comprising carbon steel and at least one alloying element selected from chromium, vanadium, manganese, tungsten and niobium wherein the microstructure is substantially bainitic.
What is further needed is an improved coating blade for use in coating operations.
What is further needed is a coating blade that exhibits high straightness, low wear and is capable of being bent around a small radius,
What is further needed is an improved coating blade that has a good working life.
What is further needed is a coating blade that minimizes machine downtime.
What is further needed is a coating blade that is economical in cost.
What is further needed is a coating blade comprised of steel wherein the steel microstructure is substantially all in bainitic form.
What is further needed is a coating blade comprised of high carbon steel having a bainitic microstructure wherein the carbon content is generally within the range of 0.70% to 1.25% carbon by weight.
What is further needed is a coating blade comprising carbon steel and at least one alloying element selected from chromium, vanadium, manganese, tungsten and niobium wherein the microstructure is substantially bainitic.
What is further needed is an improved creping blade for use in paper making operations.
What is further needed is an improved creping blade that has a good working life.
What is further needed is a creping blade that minimizes machine downtime,
What is further needed is a creping blade that is economical in cost.
What is further needed is a creping blade comprised of steel wherein the steel microstructure is substantially all in bainitic form.
What is further needed is a creping blade comprised of high carbon steel having a bainitic microstructure wherein the carbon content is generally within the range of 0.70% to 1.25% carbon by weight.
What is further needed is a creping blade comprising carbon steel and at least one alloying element selected from chromium, vanadium, manganese, tungsten and niobium wherein the microstructure is substantially bainitic.
What is further needed is a bainitic steel strip having very high straightness arid low wear.
What is further needed is bainitic steel strip having a camber of about 0.040 inch per ten feet of length and, preferably, 0.024 inch per ten feet of length.
What is further needed is bainitic steel strip having a high straightness, low wear and a hardness range of 48-60 Rockwell C with little brittleness.
What is further needed is an improved process for producing bainitic steel strip.
What is further needed is an improved process for producing bainitic steel strip that overcomes the problem of bearing failure during production of the bainite
What is further needed is a printing process that uses at least one of a bainitic steel doctor blade, bainitic rule die knife, bainitic steel creping blade or bainitic steel coating blade.
What is further needed is a process for flexographic printing using at least one of a bainitic steel doctor blade, bainitic steel rule die Life, bainitic steel creping blade or bainitic steel coating blade.
What is further needed is a process for gravure printing using bainitic steel components.
What is further needed is a process for pad printing using bainitic steel components.
What is further needed is a process for electrostatic printing, glue application, die cutting, coating and paper making using bainitic steel components,
What is further needed is a high temperature bearing assembly used in the process of producing bainite strip steel wherein salt is used as a protective agent against oxidation and/or deterioration of the bearing assembly.
What is further needed is a high temperature bearing assembly used in the process of producing bainite strip steel wherein tin is used as a protective agent against oxidation and/or deterioration of the bearing assembly.
What is further needed is heat treating equipment used in the process for producing bainitic strip steel wherein salt is used as a protective agent against oxidation and/or deterioration.
What is further needed is heat treating equipment used in the process for producing bainitic strip steel wherein tin is used as a protective agent against oxidation and/or deterioration.
Other objects of the present invention will become apparent to those of ordinary skill in the art.
The present invention is directed to doctor blades, rule die knives, creping blades and to coating blades comprised of bainitic steel and to a method for producing bainitic steel strip. The present invention also provides for printing and other processes that use bainitic components and, bainite production processes that preserve the useful life of anti-friction bearings used therein.
The present invention is accomplished by using bainitic steel that exhibits superior straightness and wear properties and is also bendable around small radii. The bainitic steel is produced by continuously heat-treating steel strip steel under tension in a manner to produce a bainitic microstructure of a specific hardness, strength and microstructure. The initial steel must have a specific microstructure to maximize the wear properties and the straightness of the final product. Tension must be controlled so that elongation minimizes the size reduction of the strip.
The process of the present invention comprises the steps of, annealing a carbon steel resulting in a microstructure of the steel having a dispersion of carbides in a ferritic matrix; cold rolling the annealed steel; cleaning the cold rolled steel to remove oil and dirt; bridle roll and/or friction braking the cleaned steel to increase strip tension; austenitizing the steel, submersing the austenitized steel into a quenchant; removing excess quenchant; and isothermally transforming the austenitized steel into bainite.